Pretreatment of Bagasse
Sulfuric acid is the most commonly used acid in the pretreatment of sugarcane bagasse [95], but other mineral acids such as hydrochloric, nitric acid [96] and phosphoric acid [97] can also be used for this purpose [98]. Additionally, the usage of acetic acid along with hydrogen peroxide has been reported as a way to remove lignin prior to enzymatic hydrolysis of sugarcane bagasse. Other methods reported in the literature for pretreatment are: alkaline methods [99], alkaline pretreatment followed by steam explosion [100], and acid pretreatment under sonication [101].
Dias and coworkers have proposed an organosolv pretreatment process-based technology for saccharification of sugarcane bagasse to fermentable sugars. A three-step hydrolysis involving pre-hydrolysis of hemicellulose, organosolv delignification and cellulose hydrolysis is the key to this technology [102]. The main advantage of this method is that this method does not require the enzymatic hydrolysis step. The organic solvent normally used in the process is ethanol and can be recovered and reused. The key steps in the process are outlined in the flow chart diagram in Figure 3.2. [102].
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Figure 3.2 Flow chart diagram of the organosolv process for sugarcane bagasse with dilute acid hydrolysis in separate steps. |
This method has been tested in a demonstration unit for the production of ethanol in 5000 L/day scale in Brazil [103]. According to their experience, this configuration allows the removal of pentoses prior to the extreme conditions of cellulose hydrolysis, that diminishes pentose decomposition and consequent production of fermentation inhibitors such as furfural, leaving cellulose and lignin fractions unaltered [104]. Since the hydrolysis reaction uses H2SO4 as catalyst after lignin removal, this configuration would also allow the production of lignin without traces of sulfates, so lignin recovered could be burnt in conventional boilers with no corrosion risks. This configuration seems to be a promising strategy for processing of low lignin, high cellulose feedstocks like sugarcane bagasse. Furthermore, this configuration allows the conversion of lignin and hemicellulose into valuable coproducts, which off-sets the costs of feedstock pretreatment [105]. In a biorefinery in Brazil, in addition to sugarcane juice first generation bioethanol, a fraction of the bagasse waste produced in the mills is used as raw material for cellulosic ethanol production using the hydrolysis process.